30 research outputs found
Energy- and flux-budget (EFB) turbulence closure model for the stably stratified flows. Part I: Steady-state, homogeneous regimes
We propose a new turbulence closure model based on the budget equations for
the key second moments: turbulent kinetic and potential energies: TKE and TPE
(comprising the turbulent total energy: TTE = TKE + TPE) and vertical turbulent
fluxes of momentum and buoyancy (proportional to potential temperature).
Besides the concept of TTE, we take into account the non-gradient correction to
the traditional buoyancy flux formulation. The proposed model grants the
existence of turbulence at any gradient Richardson number, Ri. Instead of its
critical value separating - as usually assumed - the turbulent and the laminar
regimes, it reveals a transition interval, 0.1< Ri <1, which separates two
regimes of essentially different nature but both turbulent: strong turbulence
at Ri<<1; and weak turbulence, capable of transporting momentum but much less
efficient in transporting heat, at Ri>1. Predictions from this model are
consistent with available data from atmospheric and lab experiments, direct
numerical simulation (DNS) and large-eddy simulation (LES).Comment: 40 pages, 6 figures, Boundary-layer Meteorology, resubmitted, revised
versio
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The influence of the atmospheric boundary layer on nocturnal layers of noctuids and other moths migrating over southern Britain
Insects migrating at high altitude over southern Britain have been continuously monitored by automatically-operating, vertical-looking radars over a period of several years. During some occasions in the summer months, the migrants were observed to form well-defined layer concentrations, typically at heights of 200-400 m, in the stable night-time atmosphere. Under these conditions, insects are likely to have control over their vertical movements and are selecting flight heights which are favourable for long-range migration. We therefore investigated the factors influencing the formation of these insect layers by comparing radar measurements of the vertical distribution of insect density with meteorological profiles generated by the UK Met. Office’s Unified Model (UM). Radar-derived measurements of mass and displacement speed, along with data from Rothamsted Insect Survey light traps provided information on the identity of the migrants. We present here three case studies where noctuid and pyralid moths contributed substantially to the observed layers. The major meteorological factors influencing the layer concentrations appeared to be: (a) the altitude of the warmest air, (b) heights corresponding to temperature preferences or thresholds for sustained migration and (c), on nights when air temperatures are relatively high, wind-speed maxima associated with the nocturnal jet. Back-trajectories indicated that layer duration may have been determined by the distance to the coast. Overall, the unique combination of meteorological data from the UM and insect data from entomological radar described here show considerable promise for systematic studies of high-altitude insect layering
Energy- and flux-budget turbulence closure model for stably stratified flows. Part II: the role of internal gravity waves
We advance our prior energy- and flux-budget turbulence closure model
(Zilitinkevich et al., 2007, 2008) for the stably stratified atmospheric flows
and extend it accounting for additional vertical flux of momentum and
additional productions of turbulent kinetic energy, turbulent potential energy
(TPE) and turbulent flux of potential temperature due to large-scale internal
gravity waves (IGW). Main effects of IGW are following: the maximal value of
the flux Richardson number (universal constant 0.2-0.25 in the no-IGW regime)
becomes strongly variable. In the vertically homogeneous stratification, it
increases with increasing wave energy and can even exceed 1. In the
heterogeneous stratification, when IGW propagate towards stronger
stratification, the maximal flux Richardson number decreases with increasing
wave energy, reaches zero and then becomes negative. In other words, the
vertical flux of potential temperature becomes counter-gradient. IGW also
reduce anisotropy of turbulence and increase the share of TPE in the turbulent
total energy. Depending on the direction (downward or upward), IGW either
strengthen or weaken the total vertical flux of momentum. Predictions from the
proposed model are consistent with available data from atmospheric and
laboratory experiments, direct numerical simulations and large-eddy
simulations.Comment: 37 pages, 5 figures, revised versio